In curbing grain sales to the Soviet Union to protest its invasion of Afghanistan, President Carter dealt a potential $110 million blow to the economy of Iowa -- whose voters on Jan. 21 will choose the first delegates in the 1980 presidential contest.

"There'll be a terrific howl go up," said Woodrow W. Diehl, chairman of the Farmers for Carter in 1976, who is seen in a current television commercial for the president. "But knowing the man as I do, if it means losing the caucuses in Iowa, he's goingto do what's right for the nation."

The immediate reaction here was that Carter -- in his sternly worded response to the Soviet attack -- had stolen a march on his Republican rivals, who had expected to criticize his "inaction" in their televised debate here tomorrow night.

But by canceling 17 million metric tons of grain deliveries to the Soviets, he gave the GOP hopefuls a chance to accuse him of breaking a pledge he made to Iowa formers in 1976, when he said at the state fair he would "end embargos once and for all."

Carter's television speech had barely ended when Sen. Bob Dole (R-Kan.) said he had "broken faith with the American farmer" while sending up a "smokescreen" to conceal his continued inability to gain the release of the U.S. hostages in Iran.

"Having devastated the American farmer," Dole said, "President Carter now will call on the U.S. taxpayer to bail out the farmer. It's senseless. nThey [the Soviets] willl just buy their grain somewhere else, and we will lose the market."

Carter said in his speech that other grain-exporting countries had promised not to cash in on the U.S. decision by expanding their sales to the Soviet Union. He also promised to "minimize any adverse effect" on the farmers by a multibillion-dollar government purchase and exports subsidy program.

Diehl, who played host to Carter in the 1976 camapaign, said, "I have enough faith in Carter . . . that he isn't going to wipe out all the grain farmers, but there's no way this is going to help him here. The trouble is the average farmer doesn't understand the precarious situation we're in."

Iowa Secretary of Agriculture Robert Lounsberry estimated yesterday that a cutoff of grain shipments to the Soviet Union would cost the state's farmers about $165 million. Carter's proposal would let about one-third of the contracts stand, leaving the potential loss to the Iowa economy at about $110 million.

The injection of the grain sales issue represents another dramatic turn in the battle between Carter and Sen. Edward M. Kenedy (D-Mass.) in the Jan. 21 Iowa caucuses. Since the Iran hostage crisis started and Kennedy entered the race, Carter had come from 2 to 1 deficit in the Iowa Poll to a point where he was a slight favorite.

In preempting the networks 24 hours before six GOP candidates hold their debate here, Carter laid down a defense against the expected charge that he was temporizing in the face of Soviet aggression.

White House aides said the imminence of the GOP debate was considered but was not a major factor in the timing of the Carter speech.

The aides said Carter had pressed them for the economic analysis he needed to back up his anti-Soviet decisions, but said he did not want to debate with them the political implicataions of the move.

Tonight's speech seemed likely to supercede, and to some extent silence, the criticism that has been heard here of Carter's decision a week ago to cancel his participation in a scheduled Monday night debate here with Kennedy and California Gov. Edmund G. (Jerry) Brown Jr.

Earlier today, Mondale had conceded that the president's supporters were "a little bit concerned" about his decision, which had the effect of continued criticism, particularly in the pages of the Des Moines Register, the sponsor of the event.

But Mondale drew applause from Carter rallies in Waterloo, Cedar Rapids and Dubuque when he said that most people understood that Carter had to work "round-the-clock" on the crises in Iran and Afghanistan.

It was in August 1976 that Carter told an Iowa State Fair audience he would never do what President Ford had done: cancel grain shipments overseas when farmers had planted in expectation of the export sales. Two days later, Aug. 26, amended the statement to say that he would cut off shipments to foreign markets only in response to "a catastrophe of almost unimaginable degree that threatened famine in the United States."

In an interview last month with the Register, he repeated his view that America's food surpluses should be used "in a beneficial way and for peace, not to punish other people."

Two days ago, the pro-Carter chairman of the Democratic National Committee, John C. White, a long-time Texas commissioner of agriculture, told reporters in Washington that the Soviet Union could adjust to a grain embargo more easily than could American agriculture. "The impact on the Russians would be minimal," White said, "and the impact on our economy would be major."

Iowa Gov. Robert D. Ray (R) said he understood the president's need "to send a message to Russia," but he said that farmers are going to be "very skeptical . . . uneasy and concerned" about having agriculture singled out for use as a diplomatic weapon.

Ray expressed his own skepticism that storage facilities would be available for crops the government would buy to bolster prices or that gasohol could consume a significant share of the grain that otherwise would have gone to the Soviet Union.

Even before Carter's decision was announced, Iowa farm leaders were denouncing the threat of an embargo.

However, Devon Woodland, president of the National Farmers Organization, said he thought that, even though the Soviet grain market was vital to American farmers, "if you have an enemy, you don't feed him and make him strong." house at 12331 Tilbury La. for $80,500 to Mr. and Mrs. Harvey C. Galowin